1. Field of the Invention
The subject invention relates to electrooptical systems, to light gate structures, assemblies, modules and elements, and to method of providing or preparing same.
2. Disclosure Statement
This disclosure statement is made pursuant to the duty of disclosure imposed by law and formulated in 37 CFR 1.56(a). No representation is hereby made that information thus disclosed in fact constitutes prior-art inasmuch as 37 CFR 1.56(a) relies on a materiality concept which depends on uncertain and inevitably subjective elements of substantial likelihood and reasonableness, and inasmuch as a growing attitude appears to require citation of material which might lead to a discovery of pertinent material.
Various electrooptical light gate systems have been proposed in diverse fields of utility. For instance, an article by J. Thomas Cutchen et al., entitled Electrooptic Devices Utilizing Quadratic PLZT Ceramic Elements, published in 1973 WESCON TECHNICAL PAPERS, Vol. 17, part 30, pp. 30/2 et seq., and an article by the same authors entitled PLZT Electrooptic Shutters: Applications, APPLIED OPTICS, Vol. 14, No. 8 (Aug. 1975), pp. 1866 et seq., describe electrooptic ceramics and devices employing transparent lanthanum-modified lead zirconate titanate (PLZT), and applications thereof, including page composers, display devices, eye protection devices, industrial welding protection, large aperture photographic shutters and variable density filters.
Reference is also made to the extensive bibliography of these two articles, hereby incorporated by reference herein.
Facsimile apparatus for writing and reading mechanically moving documents with an electronically controllable switching mask plate, disposed between polarization filters and consisting of a material containing mixed crystals of lead zirconate and lead titanate, and doped with lanthanum, and provided with aligned electrodes, was proposed in U.S. Pat. No. 3,930,119, by Rolf Schmidt et al., issued Dec. 30, 1975.
That proposal contemplated use of a switch mask plate having a large number of electrodes arranged in line with one another, or then employment of a plurality of ceramic plates provided with electrodes and stacked so that light beams could be masked according to a raster between the electrodes of the individual plates.
A further proposal is apparent from British patent specification No. 1,534,027, by Battelle Memorial Institute, published Nov. 29, 1978. That proposal employs light modulating elements which are arranged side by side, and each of which has the form of an electrooptical shutter. Electrode arrangements are also shown which have comblike electrode configurations, or similar arrangement with a grounded serpentine common electrode located on both major sides of a ceramic plate, with spatial coincidence or registration existing among corresponding electrodes on the two substrate faces.
Improved light gate utilization methods and apparatus, with special optical systems, were disclosed in German patent Publication No. 28 09 997, filed by the subject assignee, published Sept. 21, 1978, and herewith incorporated by reference herein.
For many applications, including oscillographs and facsimile apparatus, it is desirable to have elongate light gate structures having a length of more than twenty centimeters or preferably in the foot (about 30 cm) range. To build a shutter array of this length without visible gaps and without distortion in linearity it has been proposed to deposit PLZT or similar material on a suitable compatible substrate by sputtering. Matching of the thermal expansion of the substrate and of the ferroelectric deposit becomes, however, a major problem at larger lengths. A mismatch in thermal expansion causes strain bias in the deposited ferroelectric film, affecting the rotation of the polarization vector of the transmitted light.
Another method of manufacturing ferroelectric ceramic chips is by hot-pressing and sintering cylindrical slugs which are subsequently cut into wafers or chips and optically polished. Production processes for practising this method, however, impose limits on the maximum size of chips which can be economically manufactured. In practice, this maximum size is several times smaller than the above mentioned desired lengths.
In a similar vein, ferroelectric ceramic chips or other substrates of electrooptically active material are very strain and temperature sensitive. They also are vulnerable to light-induced birefringence, affecting light transmission and quality of performance.
Mounting of ferroelectric ceramic substrates also has been a problem in terms of differential thermal expansion between substrate and mounting structure. Piezoelectric effects in the light gate material and dimensional changes resulting therefrom also have caused problems at the substrate/mounting structure interface.
Interfacing the electrodes on the light gate substrate with the driving electronic circuitry has given rise to various problems, especially when thin film systems, such as employed for providing the driving electrodes for the light gates, are desired to be interfaced with thick film systems of which the driving electronics could be most economically manufactured.
In practice, these problems are compounded by the inevitable generation of heat by the electronic circuits and equipment driving the light gate arrays. Electrooptical materials are particularly vulnerable to performance degradation by induced temperature gradients and by the temperature exposure itself.
These problems are also compounded by the need to mount light polarizing filters and lenses adjacent the light gate structure in practical systems.
Further problems stem from the vulnerability of electrooptical systems and their performance to dust and other contaminants.
In U.S. Pat. No. 3,873,187, by Robert E. Brooks, issued Mar. 25, 1975, it has been proposed to cut grooves through a layer of PLZT or other electrooptically active material and into a supporting substrate and to form electrodes by placing wires and conductive paste into these cut grooves. Where it would not be possible to form a single strip of light gate arrays in a single desired length, that reference suggests disposing several individual rectangular strips end to end, care being taken that they precisely butt each other where saw cuts will occur.
One of the more visible problems of such a proposal is that a provision of electrode-receiving grooves with a saw or by means of a similar mechanical machining process practically limits the electrode configuration to straight lines. Such straight lines typically have to extend across the full width of the PLZT layer or the substrate, rendering it difficult, if not practically impossible, to provide greater center-to-center distances between adjacent electrode terminals than the center-to-center spacing between the electrodes to which these terminals are connected. Also, saw cutting or similar machining inevitably causes chipping of the edges and of the surface, internal stresses near the cut edges and mechanical weakening of the PLZT layer or of the substrate due to the hotch effect of the cut grooves.
Another problem that has so far beset electrooptical light gate systems, such as PLZT light shutters or gate arrays, may be designated as "light induced birefringence." Such undesired birefringence occurs, for instance, when light gates are illuminated at high intensity while electric fields are applied to the gates, such as to place the gates in their ON condition. In short, light induced birefringence degrades the performance of light gates, and particularly their light transmitting capability.
While no limitation to or dependence on any particular theory is intended, it may be observed that a cause of light induced birefringence is believed due to photoexcitation of electrons in the illuminated region where electrons can reach the conduction state and drift under the influence of the applied field. Such photoinduced charge carriers are being trapped in the dark regions of the electrooptical material layer; particularly in close proximity to the edges of the electrode deposits. In brief, the trapped photoinduced charge carriers generate a space charge field which acts against the field applied to the electrodes and gating regions. The localized charge at the boundary of the illuminated area grows at a rate proportionate to the difference of free carrier concentration in the illuminated and in the dark areas. By way of example, up to 45% decrease in light transmission was observed with PLZT light gate arrays after 90 minutes of intense illumination.
Especially with PLZT light gate systems, the rule of thumb has developed that the interelectrode spacing, that is, the spacing between immediately adjacent electrodes, should not be smaller than the thickness of the layer of electrooptically active material. Such relationship, in effect, provides a favorable distribution of the applied electric fields throughout the thickness of the electrooptically active material, thereby reducing the above mentioned light induced birefringence effect. However, the latter rule of thumb places severe restrictions on a reduction of interelectrode spacing for a given thickness of the electrooptically active substrate or layer. As a result, existing systems are either subject to light induced birefringence at a detrimental rate or suffer a severe limitation on the attainment of desirable properties, such as an achievement of high resolution.
Prior proposals, such as those contained in the above mentioned Schmidt et al. patent or in U.S. Pat. No. 3,799,647, by Victor Luft, offer no solution of this serious problem.